1. Field of the Invention
The present invention relates to a semiconductor device provided with a trench element isolation film and a method for fabricating the same, and more particularly to a semiconductor device with a trench element isolation structure having a planarized element isolation film formed in a trench of a small width provided at the boundary between an active region and a field region and with a thick oxide film selectively formed on the field region upon forming a gate oxide film and a method for fabricating the semiconductor device.
2. Description of the Prior Art
In fabrication of highly integrated semiconductor devices, conventionally, the local oxidation of silicon (LOCOS) process has been used to form element isolation films for isolating active elements. However, such an element isolation film formed using the LOCOS process defines a reduced active region due to an excessive bird's beak and has an insufficient depth, thereby exhibiting a degraded element isolation function.
FIG. 1 is a layout of masks arranged in fabrication of a metal oxide semiconductor field effect transistor (MOSFET) formed with an element isolation film using the conventional LOCOS process. In FIG. 1, the reference characters A and B denote an element isolation mask and a gate electrode mask. In this case, a region defined inwardly of the element isolation mask A is an active region C whereas a region defined outwardly of the element isolation mask A is a field region D where the element isolation film is formed.
FIG. 2 is a cross-sectional view taken along the line I--I of FIG. 1, showing the formation of element isolation film in accordance with the conventional LOCOS process and the formation of MOSFET at the active region. As shown in FIG. 2, an element isolation film 2 is formed at a field region on a semiconductor substrate 1 by use of the element isolation mask shown in FIG. 1. At an active region on the semiconductor substrate 1, a gate oxide film 3 and a gate electrode 4 are formed. Thereafter, impurity ions of a conduction type different from that of the substrate 1 are implanted in the active region, thereby forming source/drain electrodes 5.
FIG. 3 is a cross-sectional view taken along the line II--II of FIG. 1, showing the formation of element isolation film in accordance with the conventional LOCOS process and the formation of MOSFET at the active region. As shown in FIG. 3, the gate electrode 4 extends laterally such that it overlaps with the element isolation film 2.
Where formation of the element isolation film is achieved using the above-mentioned conventional LOCOS process, the active region is reduced due to an excessive bird's beak. Moreover, the element isolation film has a small depth buried in the semiconductor substrate. As a result, the function of the element isolation film to isolate elements formed at the active region from each other is degraded, thereby causing a leakage current to be generated.
In order to solve this problem involved when the element isolation film is formed using the conventional LOCOS process, there has been proposed formation of an element isolation film having a trench structure.
However, such a trench element isolation film has a problem that where a trench having a large width is formed, the element isolation film buried in the trench is hardly planarized.